The Role of Auxins in Root‐Knot Nematode‐Induced Growth on Excised Tobacco Stem Segments

Abstract

AbstractRoot‐knot nematodes, Meloidogyne incognita, induced lumps of callus tissue on the cambial surfaces of peeled tobacco stem segments cultured in vitro. Except for a layer 1 to 3 cells thick, callus was limited to the basal ends of control segments. Indole‐3‐acetic acid (IAA) applied in agar blocks to the centers of stem segments, when it had any effect on the cambial surface, induced streaks of callus extending from the blocks toward the basal ends of the segments. IAA in agar blocks also increased callus growth at the basal ends of the segments, increased the growth of pith on the undersides of the segments, promoted root initiation, but inhibited bud initiation. Nematodes produced none of these effects, nor did they change the type of organs induced by various concentrations of IAA in the medium. Callus tissue did grow on the cambial surface of stem segments surrounding agar blocks containing 2,3,5‐triiodobenzoic acid, an inhibitor of polar auxin transport.Paraffin sections showed that the nematodes were confined to the callus tissue on the cambial surfaces of the segments. Except for occasional syncytia and areas of cell division, nematode‐induced callus was composed of thin‐walled, irregularly shaped cells arising from the cambium.Differences between the responses of tobacco stem segments to root‐knot nematodes and IAA‐agar blocks indicate that auxins were not freed from the plant tissue nor secreted by the nematodes. Instead, it is suggested that nematodes enabled the tissue to retain and use endogenous auxins that otherwise would have been transported to the basal ends of the segments.